This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Isotope-Assisted Differential Metabolomics: a New Strategy for High Throughput Analysis of Global Metabolic Profiles. Over the past 10 years, a considerable effort has been devoted to developing high throughput methods for profiling metabolites in biological samples. Although we can detect molecules at concentrations as low as 10-18 molar with mass spectrometry (MS) and can differentiate between closely related species using nuclear magnetic resonance (NMR), sample complexity has severely limited the performance of these techniques in metabolomics studies. We have developed a new heavy-isotope based strategy that minimizes problems associated with traditional methods and extends the scope and reproducibility of global metabolic profiling. We are currently using this approach to investigate the mechanism of drought resistant in genetic knockouts of Arabidopsis thaliana and to elucidate the regulation of metabolic flux in human erythrocytes.